Acoustic resistances made of woven or etched mesh or screen are well known in the art. In such materials, the apertures can be made small, a condition necessary to produce acoustic impedances in which dissipative, resistance parameter substantially exceeds the reactive inertance parameter. Common in the prior art, the etched mesh or screen is cemented or clamped over an aperture to obtain an acoustic impedance having a relatively pre-determined value.
When metal screens or perforate mesh are used, it is relatively simple and inexpensive to form and shape such materials into useful devices or plugs for insertion into acoustic apparatus. Such devices are relatively sturdy but if once distorted, it is difficult to restore the devices to their original shape.
Accordingly, it is a feature and purpose of this invention to provide acoustic devices that are durable and can be conveniently handled and cleaned, inserted and removed from their operational locations without damage to the device.
Other types of acoustic plugs comprise plastic or metal bushings having a hole of selected size to provide a desired degree of closure to modify the frequency response in accordance with the wearer's requirements.
However, it has been found that when a single hole, or even a few holes formed in a concentrated area, are used to obtain the resistance, the sound, in order to pass through these holes, must converge to this small area. This action contributes an inertance component to the impedance limiting the quality of the acoustic element as a resistive element. If holes are produced in a less concentrated area, such as over the surface of the mesh, the movement of air in the sound is not forced to store as much energy in inertia to converge to the holes that produce the frictional component of impedance, thereby providing the result that the inertance component of the impedance is lower and the acoustic element provides a better resistance.
Also, if high intensity sound such as would occur at the outlet of a receiver is channeled through a single hole, or even through a few holes, there is a tendency for unwanted turbulence to develop.
Accordingly, it is another feature and purpose of the present invention to provide an acoustic plug which provides a selected acoustic impedance while developing no disruptive turbulence.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description as illustrated in the accompanying drawings wherein: